Ruthenium complexes of formula A are known from WO 02/14376 A2 and are described as active, air-stable and recoverable metathesis catalysts. Catalysts of this kind which have an even greater activity than A have also become known (Angew. Chem. 2002, 114, No. 5, 832-834; Angew. Chem. 2002, 114, No. 13, 2509-2511; Angew. Chem. 2002, 114, No. 21, 4210-4212), and are described by the formulae B, C and D.

The improvement in the activity of B, C and D compared with A is due to steric and electronic effects of the substituents at the phenyl nucleus of the isopropoxy-phenylmethylene ligand. It has now been found, surprisingly, that a major increase in the activity of metathesis catalysts of type A can be achieved by making particular changes to the aliphatic moiety of the ether group.